Genetic Demography of Tribal Populations -- The study of genetic structure in human tribal populations is well advanced, but most of the work to date has been more descriptive than predictive. Most of the parameters used in the theory of population genetics (eg., effective population sizes, migration rates, population growth rates, and genotypic fitness values) are really demographic, social and ecological variables, rather than constants. If we are ever to understand human evolution in more than descriptive and retrospective terms, we must move the study of genetic structur closer to its demographic, social and ecological underpinnings than has been the norm. The present proposal describes a series of studies on the relationship between demographic, social and ecological processes and genetic variation in two related and interacting tribal populations, the Gainj and Karam of highland Papua New Guinea. Particular attention is directed to: (1) the ecological determinants of demographic structure and population size and their genetic consequences (2) the effects of population subdivision on the genetic differentiation of local groups over time and space, and (3) the ecological causes and genetic consequences of gene flow among local groups. Since human tribal genetic structure is similar to the subgroup organization of many other social mammals, this research is also of very wide evolutionary significance.